UtilibyWritten by the Utiliby editorial team · Last updated October 17, 2026 · Rules and formulas reviewed October 2026
What this free BMI calculator does
Body mass index, or BMI, is a quick way to compare your weight with your height. Doctors, health surveys and insurers use it as a first screening step, and millions of people look it up every month. A BMI calculator does the arithmetic for you and tells you which band the result falls in.
This free online tool does more than return one number. It works in metric or US units and applies the standard World Health Organization adult bands or the lower Asian cut-offs. A coloured scale shows where you sit. You also get your healthy weight range for your height, how much you would need to gain or lose to reach it, BMI Prime, the Ponderal Index and, if you enter a waist measurement, your waist-to-height ratio. A goal-weight tab calculates the BMI at a target and the time needed at a weekly pace, and a height chart lists healthy ranges for common heights. Your numbers never leave your browser.
How to use the BMI calculator
- Choose your units. Metric uses centimetres and kilograms. US uses feet, inches and pounds.
- Enter height and weight. The result updates as you type.
- Add optional details. Age and sex unlock the body fat estimate, and a waist measurement gives the waist-to-height ratio.
- Pick the cut-offs. Tick the Asian box if those apply to you.
- Read the scale. The marker shows your band, and the text gives the healthy range and the change needed to reach it.
- Plan a goal. Use the second tab to see the BMI at a target weight.
The BMI formula and an example
BMI equals weight in kilograms divided by height in metres squared. Take a person who weighs 78 kg and is 1.72 m tall. Height squared is 2.9584. Dividing 78 by 2.9584 gives 26.4. In US units the formula is 703 times weight in pounds divided by height in inches squared, which gives the same answer. The formula was devised by the Belgian mathematician Adolphe Quetelet in the 19th century and was renamed body mass index in 1972.
| Height | Weight | BMI | Band |
|---|---|---|---|
| 1.60 m | 50 kg | 19.5 | Healthy |
| 1.70 m | 85 kg | 29.4 | Overweight |
| 1.80 m | 70 kg | 21.6 | Healthy |
| 1.75 m | 100 kg | 32.7 | Obese class I |
| 1.65 m | 45 kg | 16.5 | Moderate thinness |
BMI categories explained
The World Health Organization groups adult BMI as follows. Under 16 is severe thinness, 16 to 16.9 moderate thinness and 17 to 18.4 mild thinness. 18.5 to 24.9 is the healthy range. 25 to 29.9 is overweight, also called pre-obese. From 30 the bands are obese class I (30 to 34.9), class II (35 to 39.9) and class III (40 and above). These bands describe populations, and they are a starting point for a conversation about health, not a verdict on a person.
Asian BMI cut-offs
Research found that in many Asian populations the risk of type 2 diabetes and heart disease rises at lower BMI than in European populations. A WHO expert consultation in 2004 therefore described action points at 23 and 27.5, while keeping the international bands for comparison. Under this approach a BMI of 23 to 27.4 signals increased risk and 27.5 or more high risk. Some countries have set their own cut-offs, for example Japan at 25 for obesity. Tick the box to use 23 and 27.5, and ask your doctor which guidance applies to you.
What BMI cannot tell you
BMI uses only weight and height. It cannot tell muscle from fat, where fat is stored or how fit you are. A muscular athlete can have a BMI above 25 with low body fat. An older adult who has lost muscle can have a normal BMI and a high fat share. BMI is not used for children in the adult bands, for pregnant women, or as the only measure for people with certain medical conditions. For a fuller picture, look at waist size, blood pressure, blood tests, activity and diet, and discuss them with a health professional.
Waist-to-height ratio, BMI Prime and Ponderal Index
The waist-to-height ratio divides your waist by your height. A ratio under 0.5, meaning a waist less than half your height, is a simple guide used in UK guidance because fat around the middle carries more health risk than fat elsewhere. BMI Prime is BMI divided by 25, so a value of 1.05 means 5 percent over the upper healthy limit. The Ponderal Index, weight divided by height cubed, is sometimes preferred for very tall or very short people. The calculator shows all three beside your BMI.
Planning a goal weight
The second tab turns a goal into numbers. Enter a target weight and the tool shows the BMI at that weight and which band it falls in. Enter a weekly rate of change and it divides the difference by that rate to estimate the number of weeks and a date. For example, losing 8 kg at 0.5 kg a week takes 16 weeks. These are arithmetic, not advice, because the right pace depends on age, health, medicines and lifestyle. A doctor or registered dietitian can set a plan that is safe and realistic for you.
Using BMI sensibly
- Measure the same way each time. Weigh yourself in the morning, with similar clothes, and measure height without shoes.
- Look at trends. One reading matters less than the direction over months.
- Pair it with other measures. Waist size and fitness tell you more than BMI alone.
- Focus on habits. Regular movement, enough sleep and a balanced diet matter more than a number.
- Ask for help. If you are worried, see a doctor.
Privacy, accuracy and responsible use
The calculator runs in your browser and does not store or send your measurements. Results are rounded to one decimal place. The bands are those published by the World Health Organization, and the body fat estimate is a rough formula for adults. None of it is medical advice. If you have an eating disorder, are pregnant, are under 18 or have a medical condition, speak to a doctor rather than relying on a calculator.
Related free tools
Use these free online tools together with this one:
- Plan daily macros for a low-carb diet with the keto macro calculator.
- Find the best bedtime and wake-up times with the sleep cycle calculator.
- Work out a percent of weight lost or gained with the percentage calculator.
- Find your exact age in years, months and days with the age calculator.
Frequently asked questions
What is a BMI calculator?
A tool that works out body mass index, your weight divided by the square of your height, and tells you which adult weight band it falls in. This one also gives a healthy weight range, BMI Prime, the Ponderal Index and goal-weight planning.
How do I calculate BMI?
Divide your weight in kilograms by your height in metres squared. At 78 kg and 1.72 m the BMI is 78 / (1.72 x 1.72) = 26.4. In pounds and inches, multiply weight by 703 and divide by height squared.
What is a healthy BMI?
For most adults the World Health Organization lists 18.5 to 24.9 as the healthy weight range. Under 18.5 is underweight, 25 to 29.9 overweight and 30 or more obese.
What are the Asian BMI cut-offs?
A WHO expert consultation noted that health risks rise at lower BMI in many Asian populations and suggested action points at 23 and 27.5. Tick the Asian cut-offs box to use them.
Is BMI accurate?
It is a quick screening number, not a diagnosis. It does not separate muscle from fat, so athletes may read high, and older adults who have lost muscle may read normal. Waist measurement and a doctor's assessment add useful information.
Does BMI differ for men and women?
The adult BMI formula and the WHO bands are the same for men and women. Women tend to carry more body fat at the same BMI, which is why the optional body fat estimate uses sex.
What about children and teenagers?
BMI for under-18s is compared with age and sex percentile charts, such as the CDC growth charts, rather than the adult bands. This calculator shows the number but does not label the category.
What is BMI Prime?
BMI divided by 25, the upper limit of the healthy range. A value below 1.00 is under the limit and above 1.00 is over it. It is handy for comparing across scales.
What is the Ponderal Index?
Weight in kilograms divided by height in metres cubed. It is sometimes used instead of BMI for very tall or very short people, because it scales differently with height.
How is my healthy weight range calculated?
It is the weights that give a BMI of 18.5 up to 24.9 at your height. For example, at 1.72 m that is about 54.7 to 73.7 kg.
What is the waist-to-height ratio?
Waist divided by height. Guidance such as NICE in the UK suggests keeping your waist under half your height, a ratio below 0.5. Add your waist to see it.
How is the body fat estimate worked out?
With the Deurenberg formula for adults: 1.20 x BMI + 0.23 x age - 10.8 x sex (1 for men, 0 for women) - 5.4. It is a rough estimate that can be off by several percentage points, so do not rely on it.
How long will it take to reach my goal weight?
Enter a target and a weekly change. The tool divides the difference by that rate to give weeks and a date. A safe pace is personal, so ask a doctor or dietitian.
Is BMI valid during pregnancy?
No. BMI is not used to judge weight during pregnancy. Your midwife or doctor will advise on healthy weight gain.
Should I worry if my BMI is outside the healthy range?
A BMI outside 18.5 to 24.9 is a prompt to look at the whole picture, including waist, fitness, diet and family history. It is not a diagnosis, so talk to a health professional if you are concerned.
Is my data stored or sent anywhere?
No. All calculations run in your browser and nothing is stored or uploaded.
Is this BMI calculator free?
Yes. It is free to use online with no sign-up, and you can copy the result, download a CSV or print a summary.
Sources and further reading
- World Health Organization, Body mass index (BMI).
- WHO Expert Consultation, Appropriate body-mass index for Asian populations, The Lancet, 2004.
- Centers for Disease Control and Prevention, About BMI.
- NICE, Overweight and obesity management.
Sources were consulted in October 2026. Standards and rules can change, so always check the official page for your situation.